1. Field of the Invention
The present invention relates to a friction member for a vibration wave driving device, a vibration wave driving device, and an apparatus using the vibration wave driving device as a driving source.
2. Related Background Art
In general, a vibration wave motor is designed to produce circular or elliptic motion at each point on the surface of a vibration member and to fictionally drive a contact member pressed against the vibration member.
The principle of a vibration wave motor using traveling vibration waves will be briefly described below. A vibration member (stator) is formed by bonding two groups of piezoelectric elements arranged in the circumferential direction to one surface of a ring-like elastic member formed by using an elastic material such as a metal whose total length is an integer multiple of a given length xcex.
The piezoelectric elements of each group are arranged at a pitch of xcex/2 to alternately have opposite expansion/contraction polarities. The two groups have a shift of an odd integer multiple of xcex/4. Electrode films are respectively formed on the two groups of piezoelectric elements.
When an alternative voltage is applied to one group (to be referred to as an A phase hereinafter), the vibration member produces standing waves (wavelength xcex) as bending vibrations throughout the elastic member such that the middle point of each piezoelectric element of the A phase and points xcex/2 from the central point correspond to the positions of antinodes, and the middle points between the antinodes correspond to the positions of nodes.
When an alternative voltage is applied to only the other group (to be referred to as a B phase hereinafter), standing waves are produced as in the above case, but the positions of antinodes and nodes shift from those of the standing waves produced by the A phase by xcex/4.
When alternating signals having the same frequency and a temporal phase difference of 90xc2x0 are simultaneously applied to the A and B phases, standing waves produced by the two phases are synthesized to produce traveling waves (wavelength xcex) as bending vibrations which propagate in the circumferential direction of the elastic member. At this time, elliptic motion is produced at each point on the surface of the elastic member having a thickness.
If, therefore, a ring-like contact member (e.g., a rotor as a moving member) is directly pressed and brought into contact with one surface of the vibration member, the contact member receives a frictional force in the circumferential direction from the vibration member driven to rotate.
Various materials have been proposed as friction members for vibration wave motors. As a material having the above characteristics, for example, a composite material of fluoroplastic and reinforcing fiber or a polymeric material is disclosed in Japanese Patent Application Laid-Open Nos. 1-129781 and 1-206880. This material is disclosed as a material suited to a friction material for a vibration wave motor because it has a good abrasion resistance, long service life, and stable friction coefficient.
As a method of manufacturing such a fluoroplastic composite material, a method using compression molding is generally known. In this method, a fluoroplastic powder and additive are homogeneously mixed, and the mixture is charged into a mold. The mixture is generally molded into a cylindrical molded member. The molded member is sintered at a temperature equal to or higher than the melting point of fluoroplastic. The outer surface of the sintered member is cut as if it were peeled like toilet paper, thereby obtaining a sheet-like composite material. After the material is formed into a sheet, the sheet is blanked in an appropriate size. The resultant structure is bonded as a friction member to a vibration member or contact member.
When, however, the above sheet-like fluoroplastic composite material was used as a friction member for a vibration wave motor, and the motor was driven, an abnormally large abrasion loss was produced on the surface of the friction member or the bonded friction member peeled off during driving of the vibration wave motor.
According to one aspect of the invention, as a friction member used for a vibration wave driving apparatus, a friction member ensuring excellent abrasion properties for the vibration wave driving apparatus is provided, which is formed by forming a molded member mainly made of plastic by compression molding of a plastic powder and an additive, forming a sintered member by sintering the molded member, forming a sheet by cutting the sintered member in the form of a sheet, and removing a modified layer from a surface of the sheet, which is produced by cutting.
According to one aspect of the present invention, as a friction member used for a vibration wave driving apparatus, a friction member ensuring excellent abrasion properties for the vibration wave driving apparatus is provided, which is formed by forming a molded member mainly made of plastic by compression molding of a plastic powder and an additive, and sintering the molded member, the fiber member being aligned substantially perpendicular to a friction surface and having a specific gravity of not less than 80% of a theoretical specific gravity.
Other features and advantages of the present invention will be apparent from the following detailed description.